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Introduction Nostoc commune is a worldwide-occurring cyanobacterium, forming macroscopic colonies. It is quite frequent in polar regions, where it grows on wet soil and in shallow wetlands. As a nitrogen fixator, Nostoc plays an important role in the nitrogen balance of the Arctic environments. In the field, Nostoc is exposed to many stress factors influencing its physiology. Apart from freezing temperatures and long-lasting darkness in winter, desiccation is one of most important stress factor. Material and Methods The thalli of Nostoc commune were collected in Petunia Bukta, Central Spitsbergen, Svalbard. They were used for three experiments focused to (1) desiccation in the field under natural conditions, (2) desiccation in laboratory conditions, (3) laboratory experiment evaluating Nostoc responses to elevated UV-B radiation. Weight of thalli was measured in order to calculate changes in relative water content (RWC). In laboratory desiccation experiment, thallus water potential (WP) was measured by a dew point water potential meter (WP4T; Decagon Devices, USA). Two spectral indices were measured during desiccation: (1) normalized difference vegetation index (NDVI; related to the amount and hydration state of chlorophyll) and (2) photochemical reflectance index (PRI; related to the content of carotenoids). Two different chlorophyll fluorescence parameters were measured to evaluate photosynthesis during desiccation. In the field experiment, fast chlorophyll fluorescence kinetics (OJIP) was used, while effective quantum yield (?PSII) was measured in laboratory. In the experiment with supplemental UV-B radiation, the thalli were exposed to three UV-B doses: (1) 0.8 mW, (2) 1.5 mW, and (3) 3 mW for 5 d. During the experiment, chlorophyll fluorescence was measured by Handy FluorCam FC 1000-H (Photon System Instruments, Czech Republic). The measurements were made 90 min, 3 h, 6 h, 16 h, 40 h, 3 d, and 5 d after the beginning of exposition. In order to analyze their absorbance spectra, part of thalli was removed after 24 h of exposition; the rest was analyzed after the end of experiment. Results and Discussion The relationship between RWC and WP was nearly hyperbolic, with very slow decrease of WP down to RWC values around 0.2 and a rapid decrease of WP after losing 95% of releasable water. This may indicate that the polysaccharidic envelope contains high amount of water that can be released during initial stages of desiccation and help the thallus to maintain physiological activity. The OJIP curves flattened with desiccation indicating the decrease of electron transport in PS II, although the total fluorescence signals rose in medium dehydration due to thallus shrinking. ?PSII was too variable to estimate a desiccation-dependent relationship, maybe because of physical and optical properties of the thallus. The PRI rose with desiccation from -0.3 reached at full hydration to 0 in dry state. NDVI showed a curvilinear relationship with desiccation, with a mild increase during initial dehydration and a decrease in further stages of desiccation. Photosynthetic chlorophyll fluorescence parameters decreased in all levels of UV-B radiation and the decrease lasted through all the time of exposition. The absorbance spectra showed that exposed thalli contain higher amounts of UV-absorbing compounds per gram of dry weight. The absorbance rose mostly in two ranges of wavelengths – (1) in one broad peak approximately around 380 nm that can be attributed to scytonemin content and (2) in UV-C region, where two narrow peaks at 209 and 260 nm were observed. Higher doses of UV-B caused faster decrease of chlorophyll fluorescence, but the rise of absorbance was similar in all doses. That means Nostoc was sensitive even to environmentally relevant level of UV-B radiation. Further field and laboratory experiments are needed to support such conclusion. Acknowledgements The authors thank to the project CzechPolar for providing infrastructure.
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